1. Field of the Invention
The present invention relates to a solid state imaging device for use in video camera, etc. and a method of fabricating thereof(JP-A 3-174771).
2. Description of the Related Art
In general, a solid state imaging device has an opening in a photodiode portion and a metallic barrier layer covering all the charge transfer portion in order to prevent an error from occurring which is caused by light injection to the charge transfer portion.
A prior art solid state imaging device will be described in connection with the accompanying drawings.
FIG. 10 is a sectional view illustrating an example of the prior art solid state imaging device. In FIG. 10, the reference numeral 1 indicates a semiconductor substrate, the reference numeral 2 indicates a P-type region, the reference numeral 3 indicates an N-type region in a photodiode portion, the reference numeral 4 indicates an N.sup.- -type region in a vertical CCD portion, and the reference numeral 5 indicates a P.sup.++ region. The reference numeral 6 indicates a silicon oxide layer, the reference numeral 7 indicates a silicon nitride layer, the reference numeral a indicates a silicon oxide layer, the reference numeral 9 indicates a polycrystalline silicon electrode, the reference numeral 10 indicates a polycrystalline silicon oxide layer, the reference numeral 11 indicates an interlayer insulating layer, the reference numeral 16 indicates a polycrystalline silicon layer, the reference numeral 13 indicates a light-shielding layer made of tungsten or silicide thereof, the reference numeral 14 indicates an interlayer insulating layer, and the reference numeral 15 indicates a protective layer.
The semiconductor substrate 1 is an N-type silicon substrate. A P-type region 2 is provided along one main plane of the semiconductor substrate 1. Impurities are selectively diffused into the P-type region 2 on the one main plane side of the semiconductor substrate 1 so that an N.sup.- -type region 3 in a photodiode portion and an N-type region 4 in a vertical CCD portion are provided a predetermined distance apart from each other.
A silicon oxide layer 6 is provided on the one main plane of the semiconductor substrate 1. A silicon nitride layer 7 is selectively provided on the silicon oxide layer 6. A thin silicon oxide layer 8 is provided on the silicon nitride layer 7.
Provided on the silicon oxide layer 8 is a polycrystalline silicon electrode 9 which acts as a gate electrode formed by CVD method under reduced pressure. A polycrystalline silicon oxide layer 10 is provided covering the top surface and the side edge of the polycrystalline silicon electrode 9 and the side edge of the oxide layer 8 and the silicon nitride layer 7. An interlayer insulating layer 11 formed by CVD method is provided covering the polycrystalline silicon oxide layer 10.
A ground adhesion layer 16 made of polycrystalline silicon and a light-shielding layer 13 made of tungsten or silicide thereof are provided laminated on one another on the interlayer insulating layer 11.
An interlayer insulating layer 14 is provided all over the light-shielding layer 13 and the oxide layer 6. A protective layer 15 is provided on the interlayer insulating layer 14.
The operation of the solid state imaging device having the foregoing structure will be described hereinafter.
The N.sup.- -type region 3 formed in the P-type region 2 in the semiconductor substrate 1 is a photodiode portion which undergoes photoelectric conversion to generate signal charge. When a pulse voltage is applied to the polycrystalline silicon electrode 9, signal charge migrates from the N.sup.- -type region 3 to the N-type region 4 in the vertical CCD portion under the polycrystalline silicon electrode 9. Subsequently, when a pulse signal is applied alternately to a first layer polycrystalline silicon electrode (not shown) and the foregoing second layer polycrystalline silicon electrode 9, signal charge is transferred.
The light-shielding layer 13 is normally made of a high melting metal, e.g., tungsten or silicide thereof. The light-shielding layer 13 exhibits a high stress and hence an extremely poor adhesion when applied in the form of single layer. Thus, it has been a common practice to provide a ground adhesion layer made of a polycrystalline silicon interposed between the light-shielding layer and the sublayer, relaxing the stress of the light-shielding layer 13 made of tungsten or silicide thereof and hence enhancing the adhesion thereof.
The charge transfer portion, too, is covered by a light-shielding layer made of a high melting metal or silicide thereof(JP-A-7-30090).
FIG. 11 is a sectional view illustrating the portion thus covered. In FIG. 11, the reference numeral 17 indicates a ground adhesion layer made of a polycrystalline silicon. The reference numeral 18 indicates a charge transfer electrode made of a high melting metal such as tungsten or silicide thereof. These layers 17 and 18 are provided on an oxide layer 8 in this order. In other words, the charge transfer electrode 18 is attached to the oxide layer with the ground adhesion layer 17 provided interposed therebetween. Like numerals are used where the constituents are the same as those of FIG. 5.
In operation, the charge transfer electrode 18 is discriminated alternately as a first phase electrode and a second phase electrode. When a pulse voltage is applied to the second phase electrode, signal charge migrates to the N-type region 4 in the vertical CCD portion under the second phase electrode 9 shown in FIG. 10. Subsequently, when a pulse signal is alternately applied to the first phase electrode and the second phase electrode, signal charge is transferred
As mentioned above, the charge transfer electrode 18 made of a high melting metal or silicide thereof exhibits a high stress and hence an extremely poor adhesion when applied in the form of single layer. Thus, it has been a common practice to provide a ground adhesion layer 17 made of a polycrystalline silicon interposed between the charge transfer electrode 18 and the sublayer, relaxing the stress of the charge transfer electrode 18 and hence enhancing the adhesion thereof.
However, the foregoing structure is disadvantageous in that the provision of a ground adhesion layer under the light-shielding layer adds to the thickness of the ground layer of the light-shielding layer, causing the multiple reflection of light at the interface of the ground adhesion layer with the adjacent layer that gives a smear component which deteriorates the smear characteristics of the imaging device.
The thicker the thickness of the ground adhesion layer is, the amount of the light in aslant penetrate through the ground adhesion layer and reach to the photodiode is increased. In the result, the smear component is increased.
Further, since the surface step becomes larger by the thickness of the ground adhesion layer, the thickness of the shielding layer is not able to be large sufficiently. In the result, the amount of the light penetrated through the shielding layer is not able to be ignored.